An unjust manipulation such as copying or falsification is relatively easy for digitized content. In particular, in remote access, a mechanism for preventing unjust use of content, which may be generated in content transmission, that is, protecting copyright while allowing personal or home use of the content is necessary. As industry-wide standard technology regarding transmission protection of digital content, DTCP (Digital Transmission Content Protection) developed by DTLA (Digital Transmission Licensing Administrator) is exemplified.
In the DTCP, an authentication protocol between apparatuses for content transmission and a transmission protocol of encrypted content are specified. In summary, the specification defines that a DTCP-compliant apparatus should not send compressed content easy to handle to outside the apparatus in an unencrypted state, that key exchange necessary for decryption of encrypted content should be carried out according to a predetermined authentication and key exchange (AKE) algorithm, and that a range of apparatuses which perform key exchange using AKE commands should be limited. A server (Source) to be a content provision source and a client (Sink) to be a content provision destination share a key through an authentication procedure by sending and receiving AKE commands. The server and the client use the key to encrypt a transmission path to perform content transmission. Therefore, an unjust client cannot obtain an encryption key unless the unjust client succeeds in the authentication with the server and thus, the unjust client cannot enjoy the content.
Initially, the DTCP defines transmission of content on a home network using a transmission path such as IEEE 1394. Recently, as typified by DLNA (Digital Living Network Alliance), there has been an increasing trend toward distributing digitized AV content through an IP network in the home. Accordingly, with intent to distribute digital content through the IP network in the home, development of DTCP technology corresponding to the IP network, that is, DTCP-IP (DTCP mapping to IP) is in progress.
The DTCP-IP that is similar to the DTCP technology is technology in which the DTCP technology is applied to the IP network. The DTCP-IP uses an IP network as a transmission path and uses a protocol for content transmission implemented on the IP network such as HTTP (Hyper Text Transfer Protocol) or RTP (Real-Time Transfer Protocol), for transmission of encrypted content. For example, when content is transmitted according to a procedure of the HTTP, the Source becomes an HTTP server and the Sink becomes an HTTP client. TCP/IP connection for the HTTP is generated and download transmission of encrypted content is performed (however, when upload transmission is performed, the Source becomes the HTTP client and the Sink becomes the HTTP server).
The IP network has been already laid broadly. In addition, it is expected that a receiving apparatus and a reproducing apparatus corresponding to the DTCP spread in the future. Therefore, it is thought that the DTCP-IP is applied to content transmission on a wide-area network such as a WAN (Wide Area Network) as well as a home network, so that unjust distribution of content can be prevented. For example, content can be securely transmitted from a server outside the home to an apparatus corresponding to the DTCP such as a television receiver in the home. Also, it is possible to have access to content protected by copyright in a server of a home network in the home, from a remote place outside the home.
However, the current DTCP-IP (DTCP Volume 1 Specification Supplement E Revision 1.31) is intended for securing only the home use of content mainly. For this reason, in order to suppress a range to perform an authentication and key exchange procedure to a home, a round trip time (RTT) is limited to a maximum of 7 milliseconds for an AKE command and an upper limit of the number of hops (TTL: Time To Live) of an IP router is set to 3.
For example, a method of accurately measuring the round trip time in the DTCP-IP has been suggested (for example, refer to Patent Documents 1 and 2). In addition, an information communication system in which, if a maximum value of a TTL value is more than 3, an authentication procedure is terminated without executing a process of a final step has been suggested (for example, refer to Patent Document 3).
As described above, if the RTT is limited for the AKE command as in the related art, when a server in the home performs an RTT check for a receiving apparatus in the home or when the server in the home performs the RTT check for a receiving apparatus in a remote place outside the home, a threshold value cannot be satisfied. As a result, the receiving apparatus cannot receive content. Meanwhile, if the threshold value of the RTT is set to a large value, unjust distribution of the content may be generated.